[ft-devel] Arabic font

with an attached font. The third character is Sukun and it should be rendered above the previous letter.

Where is the information about the Sukuns rendering stored? I didn't find it in GSUB, there is no GPOS and no kern in that font. How can my renderer know, that this and possible some other characters / glyphs should be rendered above a previous one? Do you take that info from the Unicode standard? But how do you calculate the vertical position?